fixed xml schemas (dtd & xsd) for the language file xml format

This fixes the schema definition as *.dtd and *.xsd for the language files, which has been changed last year to make it Transifex compatible.

___Document Type Definition (DTD)___
The current SkinEngine language file including the current *.dtd can be found here:
http://pastebin.com/raw.php?i=wRjaXkr3

The current SkinEngine language file including the fixed *.dtd can be found here:
http://pastebin.com/raw.php?i=fqrVRBWA

The XML-Syntax can be checked by either copying the content of the linke above into Notepad++ (with XML Tools plugin installed) or by pasting and checking them online
at http://www.w3schools.com/xml/xml_validator.asp (at the bottom, IE only)
or http://www.xmlvalidation.com/documentation.html

The first one does not validate, while the fixed one does.

___XML Schema Definition (XSD)___
Validation can be done by using Notepad++ with the Xml Tools plugins, easily.